1. Field of the Invention
The present invention relates to a detection signal processor and, more particularly, to a detection signal processor for performing signal processing including parallel to serial conversion of plural detection signals input in parallel.
2. Description of the Related Art
In a known radiography system for medical diagnoses, radiations transmitted through an object are made to hit a radiation detector equipped with a photoelectric conversion layer that is sensitive to the radiations. Electric charges are accumulated in the radiation detector in response to the radiation dose to the detector. The electric charges are successively read out as an electric current for each unit region of readout. The current read out is converted into digital data. Thus, a digital radiation image is obtained. A radiation detection panel is also known as such a radiation detector. The radiation detection panel has a glass substrate on which a large number of TFTs (thin-film transistors) and signal lines are formed to provide a TFT active matrix substrate, the TFTs being arranged like a matrix. A photoelectric conversion layer is formed on this substrate.
A signal processing unit is connected with the radiation detector. An integrator circuit for integrating a signal charge outputted as an electric current from the radiation detector for a given period, a first amplifier circuit for amplifying the output signal from the integrator circuit, a parallel to serial converter for converting the parallel signal outputted from the first amplifier circuit into serial form, a second amplifier circuit for amplifying the output signal from the parallel to serial converter, and an A/D converter for converting the output signal from the second amplifier circuit into digital data are connected in turn to constitute the signal processing unit.
Regarding readout of electric charges from a MOS solid-state image pick-up device in which image information is stored as electric charges in the same way as in a radiation detection panel, a technique for achieving power consumption reduction and random noise reduction is disclosed in JP-A No. 2002-209149. In particular, the system is composed of an output amplifier or AGC circuit and an A/D converter, as well as an output unit. A signal that was outputted from the sensor unit of the MOS solid-state image pick-up device and has undergone offset correction in a CDS/signal holding circuit and a parallel to serial conversion is applied to the output unit. The amount of a bias current supplied to the output unit is varied with the frequency of the driving signal according to a driving mode signal.
As mentioned previously, in the technique described in JP-A No. 2002-209149, the amount of the bias current supplied to the output unit to which the signal undergoing a parallel to serial conversion is applied is varied with the frequency of the driving signal according to the driving mode signal. The signal processing unit connected with the radiation detector or the like is made up of a larger number of circuits. The system is so configured that bias currents are supplied to the circuits to operate them. Therefore, where the technique described in JP-A No. 2002-209149 is applied to the signal processing unit connected with the radiation detector or the like, it is conceivable to switch the bias currents supplied to the circuits disposed on the upstream side of the circuits for processing the signal undergone a parallel to serial conversion in an attempt to reduce noise, as well as the bias currents supplied to the circuits for processing the signal undergone the parallel to serial conversion.
In this configuration, however, when high-speed operation is required, relatively large bias currents are supplied to all the circuits of the signal processing unit. The signal processing unit connected with the radiation detector is often accommodated within the same enclosure together with the radiation detector. Meanwhile, the radiation detector may have low heatproofness. Therefore, if relatively large bias currents are supplied to all the circuits of the signal processing unit, the amounts of heat generated by the circuits of the signal processing unit increase. Consequently, the inside of the enclosure gets hot. This creates the danger that the radiation detector is damaged. Furthermore, there is a possibility that characteristic variations such as offset take place as the temperature is increased. Especially, X-ray detectors used in medical applications are required to have high reliability and high stability.
Furthermore, in the above-described configuration, in a case where low-speed operation is tolerated, relatively small bias currents are supplied to all the circuits of the signal processing unit. In this case, the amounts of heat generated by the circuits of the signal processing unit decrease, resulting in decrease in thermal noise. However, this is overcome by the effects of noise components superimposed in a signal transmission path because of decrease in the gain of the amplifier circuit. Eventually, there arises the problem that the S/N deteriorates. Low-speed operation is allowed in the signal processing unit of the radiation detector in a case where a high-definition image is outputted. Where a high-definition image is outputted, it is undesirable that the S/N of the detection signal deteriorates.